Technical articles

The unknown features of Radio Frequency
power radiation causes concern for some users,
and yet they still love cell phones!

A major concern for any designer or manufacturer of systems is, how
safe is it?

Electric coupled Passive RFID transponder systems use the principle
of the transmitter radiating power from the reader to the transponders,
and that power being used to provide the operating voltages for the transponders.
This principle requires the transfer of energy from the reader to the transponders.

The IEEE and ANSI on the United States and the European Committee for
Electrotechnical Standardisation (CENELEC) in the European Community have
investigated in depth the radiation hazards of radio frequency emmisions
with regard to human safety.

To summarise these comprehensive documentsThese standards develop guidelines for the continuous exposure of humans
to radio energy at different frequencies. Both standards develop a reference
level, that is the level at which humans are aware that radio energy is
present but which is not harmful for continuous exposure. This reference
level is regularly reverified by examination and review of current research.

The committees have then recommended an addition factor of safety
of 10 times for workers and 50 times for the general public
to make sure that exposure is not harmful.

The exposures are expressed as a power density, that is the radiated
power is dissipated over an area. The following graph summarises these
recommendations, and is based on the radiation happening from a point source
one meter away from the human. In these curves the distance was chosen
as 1 meter which is a practical distance for an RFID system. If the distance
is too short, the measurement happens in the near field zone of
the antenna which will give different measurements to what happens in the
far field zone by which time the wavefronts are radiating radially.

Curves have also been shown for a typical Trolleyponder
system should it be required to operate over a four meter range. The ERP
for the Trolleyponder system to operate
over the four meter range was then compared against the reference level
and the factor of safety determined.

The left hand scale and the grid for all power levels is logarithmic

The right hand scale which refers to the factor of safety is linear
(red line)

ERP is the effective radiated power and is determined by muliplying
the power radiated by the transmit antenna gain

The graph shows that for a 915MHz Trolleyponder system, the exposure
to a human 1 meter from the reader would be less than
125 times below the reference level determined by the safety
committees. In fact it would be even safer as the reference level is for
continuous exposure and the Trolleyponder system is only activated for
short periods to read the identities of goods in the reading zone.

Mike Marsh
13 November 1998

Note
On the 3rd May 1999 Trolley Scan announced the development of a technology
called EcoTag, specifically aimed at meeting the European restrictions
of 0.5 watts ERP for RFID systems. With EcoTag, despite the severe power
restrictions, ranges of 10 meters are possible. EcoTag power density at
1 meter is 0.04Watts/sq meter..